The invention is based on a device for rain-induced switching on and off of an electric windshield wiper motor in vehicles.
In a known switching device of this type (DE-OS 22 39 359) the so-called rain sensor is constructed as a capacitive detector. Three electrodes are arranged on the inside of the windshield in the wiping area of the wiping blades. Two electrodes are connected to an alternating voltage and constitute a capacitor, so that an electric field is formed between them. The third electrode is arranged in such a way that only a small stray field of the electric field reaches it when the windshield is dry. The sensor signal is taken off at the third electrode and applied to an adder, to which the alternating voltage is fed so as to be out of phase by 180.degree.. The amplitude of the alternating voltage is dimensioned in such a way that the resultant output signal of the adder is zero when the windshield is dry.
The glass usually used to produce the windshields has a dielectric constant .epsilon. in the order of magnitude of 8. The moisture forming on the windshield during rain (water has a dielectric constant of .epsilon..apprxeq.80) changes the configuration of the lines of flux between the two capacitor electrodes in such a way that a large field component reaches the third, so-called measuring electrode. The signal amplitude of the sensor signal accordingly increases sharply. An output signal which reaches the switch for the windshield wiper motor as a closing signal occurs at the adder. The wiper motor is switched on. When the windshield is dry again, the output signal of the adder is absent and the wiper motor is switched off.
This known switching device has the problem that the rain sensor cannot distinguish between rain and water streaks which can remain on the windshield during wiping under certain circumstances. As a result of this, the wiper is often turned off too late and the wiper blades wipe in the dry state. The automatic wiper system accordingly works in an unsatisfactory manner.